Mihoyo's Honkai Impact 3rd action RPG released on PC back in 2019 with its own launcher, absent of any official announcements to expand to other storefronts. Perhaps that won't always be the case though, as players in China discovered a mention of Steam in the client through Mihoyo's test server.
Redditor hzy980512—who also noted the Genshin Impact Epic Games Store leak back in March—highlighted a post on Keylol where a screenshot of Honkai's test client surfaced that seems to mention Steam. The actual image lists two emblems, the title on the left is where Steam is mentioned, with the name indicating it's for a "limited-time download." The badge on the right just seems like a typical reward for Honkai's fifth year anniversary, which is coming up in October. The post's title is just asking what we all want to know, is Honkai Impact 3rd coming to Steam?
The second image via the Keylol post shows the actual test client, detailing the bonuses for the first player emblem that mentions Steam. Honkai Impact 3rd has given out emblems for anniversaries in the past, and we've also seen a few for other login events, so the second title checks out. However, the post mentions the listing has already been patched out of the game, making it impossible to confirm right now.
For now, it's just a rumor, but context around Mihoyo's previous actions lend it credibility. Back in May, Mihoyo updated its Honkai Impact 3rd PC launcher to work like Genshin Impact's client. Honkai is the older of the two, having originally just launched on mobile in 2016 before making its PC debut. Genshin Impact later came to the Epic Games Store in June, making it the first western PC storefront Mihoyo supported.
Mihoyo didn't confirm the Genshin Impact launch with Epic until the week before, so it may be a while before you see anything come of Honkai's Steam rumblings, if it even happens at all. The rumor sparked comments that there's hope Mihoyo will give Genshin Impact the same treatment, making it available on both PC platforms.